The present invention essentially relates to an ultrasound therapy apparatus which delivers ultrasound waves that produce thermal and cavitation effects.
The present invention also relates to therapy apparatus employing ultrasound, fitted with a cooling device.
It is known that a high-power focused ultrasound acoustic field is able to destroy human body tissue (see PCT published applications in the name of Fry W0-89/07907 and W0-98/07909).
Dunn and Fry have also described in "Ultrasonic threshold dosage for the mammalian central nervous system" IEEE transactions, volume BME 18, pages 253-256 how this destruction process involves two effects, more specifically a thermal effect and a cavitation effect.
The thermal effect predominates when the acoustic power at the point of focus is below a determined threshold of about 150 W/cm.sup.2 at 1 MHz. This thermal effect is due to the acoustic absorption of the tissue, which converts the mechanical energy of the acoustic wave into thermal energy.
The cavitation effect becomes predominant when the acoustic power at the point of focus exceeds a threshold of 150 W/cm.sup.2. This cavitation effect is linked to the formation of microscopic bubbles of gas which explode when they reach a critical diameter with local release of appreciable amounts of energy leading to destruction of neighbouring tissue.
In order to obtain destruction of tissue exclusively by thermal effects, it is necessary for the acoustic field to be able to reach a threshold of destruction referred to as the "thermal dose". This threshold is a function of temperature reached and of the duration of application. It is thus possible to destroy tissue by application of a moderate temperature increase over a long duration of application or, on the contrary, through application of a significant temperature increase over a short period of application.
The temperature increase is directly linked to the acoustic power of the ultrasound field at the point of focus.
In the case of a moderate temperature and a long duration of application, transfer and spreading of heat energy occurs around the point of focus, notably due to thermal conduction in the medium and to blood flow, which leads to poor control of the volume being treated, which may lead to healthy zones being destroyed with a resultant impairment of the quality of treatment.
In the case of elevated temperature and a short duration of application, the acoustic power at the focal point exceeds the abovesaid cavitation threshold, with the resultant obtaining of cavitation effects having a significant destructive power. This cavitation effect is particularly important at the various interfaces that the acoustic field encounters, for example at the skin, the muscles and the walls of organs. This leads to poor mastery of tissue destruction, as the latter is not limited to the zone immediately around the focus of the transducer.